A control device for an electric motor is known (see Japanese Patent Application Publication No. 06-335277), which includes an electric power converter configured to supply AC power to an AC motor and a rotating coordinate transformation unit configured to transform a two-phase rotating axis into a three-phase axis for the purpose of controlling the electric power converter. A rotating angle of at least the AC motor detected at every predetermined sampling time is used as a coordinate transformation angle. The control device disclosed in Japanese Patent Application Publication No. 06-335277 includes a first rotating angle compensation unit configured to sum up the rotating angle of at least the AC motor detected at every predetermined sampling time and a rotating angle at a time delayed from the sampling time. Hence, the summed rotating angle is set as the coordinate transformation angle.
However, the control device disclosed in Japanese Patent Application Publication No. 06-335277 does not deal with a reduction in phase margin attributed to an inherent characteristic of a motor, and has a problem of having an unstable control system.